1. Field of the Invention
The present invention relates to a head-protecting airbag housed in an upper periphery of a window (or side window) inside of a vehicle for deployment downward to protect a head of an occupant.
2. Description of Related Art
In the prior art, JP 2004-034766 A and JP 2007-161167 A are illustrative of conventional head-protecting airbags. Each of these airbags includes a gas admissive portion that is inflatable with inflation gas by separating an inner panel and an outer panel and a non-admissive portion that admits no inflation gas and keeps the inner panel and outer panel attached together. The gas admissive portion includes a primary inflatable portion that inflates in an early stage of airbag deployment and a secondary inflatable portion (also called a sub chamber or sub inflatable portion) communicated with the primary inflatable portion. The primary inflatable portion includes a protection portion for receiving a head of an occupant and, the protection portion and the secondary inflatable portion are communicated with each other by a conduit (also called a vent hole or constricted portion) that is tubular in shape. The secondary inflatable portion is provided to absorb inflation gas from the protection portion of the primary inflatable portion at the pressure increase of the protection portion in order to reduce a reaction force of the protection portion which may otherwise be applied to the occupant's head when the protection portion receives the head. The conduit includes an inlet at a side of the protection portion and an outlet to the secondary inflatable portion, and is located proximate the lower periphery of the airbag. In order that the inflation gas is fed to the secondary inflatable portion after the protection portion completes inflation, not immediately, the conduit has a length of a certain degree and a restrained opening dimension in a direction orthogonal to the axial direction from the inlet to the outlet to some extent.
However, at deployment of such conventional airbags, the primary inflatable portion unfurls and inflates downward with the secondary inflatable portion kept uninflated, and then the lower periphery of the airbag tends to move upward as a reaction to the downward deployment. That is, immediately after the completion of inflation of the primary inflatable portion, the secondary inflatable portion as well as the conduit located proximate the lower periphery of the airbag are likely to bend upward and cave in since they are not inflated.
These events may cause a bend of, i.e. occlusion of the conduit. If the occupant's head hits the protection portion and increases the inner pressure of the protection portion with the conduit occluded, the inflation gas inside the protection portion is not fed to the secondary inflatable portion, such that the protection portion cannot avoid the increase of internal pressure.